Outcome Of Interspecific Interactions Research Articles

Landscape change shifts competitive dynamics between declining at-risk wolverines and range-expanding coyotes, compelling a new conservation focus

Conservation actions need to target ecological mechanisms of species declines to be effective, but these mechanisms are often opaque. Shifting balance of competition is one pathway to species declines. Competitive coexistence is maintained only where competitive pressures balance across space. As resources change and new resources are introduced or disappear across landscapes, so should competition outcomes. We tested how novel anthropogenic disturbances affect this competitive landscape in a region with a diverse large carnivore community. We modelled fine-resolution spatiotemporal co-occurrence of wolverine (Gulo gulo), an at-risk Nearctic facultative scavenger, and other competing carnivores. We posed hypotheses about the different interspecific interactions – competition, predation, and facilitation – that could affect wolverine distribution across a gradient of anthropogenic disturbance. We used an information-theoretic model-selection framework to weigh evidence for hypotheses about the outcomes of interspecific interactions, inferred from habitat selection in relation to co-occurrence or segregation with other carnivores. Wolverine occurrence in space and time was explained by anthropogenic disturbance features and coyote co-occurrence at sites, revealing that coyote occurrence is a synergistic factor with anthropogenic disturbance. Wolverine were generally segregated from coyotes and avoided linear features; however if wolverine and coyotes did co-occur, they were twice as likely to co-occur at sites with linear features. Thus linear features increased opportunity for coyotes—a generalist species thriving in human disturbed landscapes – to compete with wolverines. We suggest this threat of increased competition is a mechanism potentially contributing to broad-scale wolverine range recessions from increasingly disturbed areas. Landscape change manifests as more than just physical disturbances: it alters the ecological processes that structure communities. These processes contribute to declines of species that cannot adapt to the novel disturbance features. We emphasize competition as an overlooked outcome of landscape change that could inform better conservation decisions to stem species declines.

Cats, canines, and coexistence: dietary differentiation between the sympatric Snow Leopard and Grey Wolf in the western landscape of Nepal Himalaya

Understanding the dietary habits of sympatric apex carnivores advances our knowledge of ecological processes and aids their conservation. We compared the diets of the sympatric Snow Leopard Panthera uncia and Grey Wolf Canis lupus using standard micro-histological analyses of scats collected from the western complex of Nepal Himalaya. Our study revealed one of the highest recorded contributions of livestock to the diet of top predators (55% for Grey Wolf and 39% for Snow Leopard) and high dietary overlap (0.82) indicating potential exploitative or interference competition. Their diet composition, however, varied significantly based on their consumption of wild and domestic prey. Limitation in data precludes predicting direction and outcome of inter-specific interactions between these predators. Our findings suggest a high rate of negative interaction with humans in the region and plausibly retaliatory killings of these imperilled predators. To ensure the sustained survival of these two apex carnivores, conservation measures should enhance populations of their wild prey species while reducing livestock losses of the local community through preventive and mitigative interventions.

Influence of Density on Interspecific Competition Between Spathius galinae (Hymenoptera: Braconidae) and Tetrastichus planipennisi (Hymenoptera: Eulophidae), Larval Parasitoids of the Invasive Emerald Ash Borer (Coleoptera: Buprestidae).

The outcomes of interspecific interactions between parasitoids depend on a variety of factors. Understanding the influence of these factors is important for classical biological control, where the success of parasitoid releases partly depends on interactions with native and other introduced species. However, results from laboratory experiments may not always reflect those in the field, as densities may be artificially inflated. To mitigate this problem, we examined the effects of multiple densities on interspecific competition between two larval parasitoids of emerald ash borer (Agrilus planipennis Fairmaire): Spathius galinae Belokobylskij and Tetrastichus planipennisi Yang. Parasitoid species were housed individually or together at two different densities, and we measured the effects on percent parasitism and progeny production, before calculating the interaction strengths. We found no significant effects of parasitoid density on percent parasitism, but the effect of competition on parasitism generally was reduced at lower densities. However, there were significant differences in parasitism by species, with S. galinae parasitizing more larvae than T. planipennisi. There were also no significant effects of parasitoid density on the number of progeny produced by each species, though the effect of competition on progeny production was greater at higher densities. Similarly, though, there were significant differences between species in the number of progeny produced. Specifically, T. planipennisi consistently produced larger broods than S. galinae. Our findings complement existing research suggesting that competition between these two species in the field will likely be negligible.

Behavioral interactions under multiple stressors: temperature and salinity mediate aggression between an invasive and a native fish

Interspecific aggression is a critical determinant of the success and competitive superiority of many invasive over native species. While single abiotic stressors can alter aggression levels, the manner in which multiple stressors may alter the strength and outcome of interspecific interactions and hence the invasion potential of a species is still poorly understood, even though multiple stressors are prevalent in many ecosystems. Furthermore, multiple stressors may interact to produce synergistic or antagonistic effects on individual level behaviors, thereby modulating invasive-native species interactions in unexpected ways. Here we examined the effect of two key abiotic stressors in freshwater ecosystems-temperature and salinity-on interspecific aggression between the invasive eastern mosquito fish (Gambusia holbrooki) and juveniles of the native Australian bass (Macquaria novemaculeata). Under controlled laboratory conditions, individuals were exposed to low or high salinity levels (15 and 35‰), and low or high temperatures (21 and 28 °C), and the frequency of interspecific aggressive behaviors was scored. The effect of temperature and salinity on interspecific aggression was antagonistic for both M. novemaculeata and G. holbrooki. While elevated temperature promoted aggression, elevated salinity partially or entirely negated this effect. Moreover, regardless of temperature, M. novemaculeata was more aggressive than G. holbrooki under elevated salinity. In addition to this, the native displayed more aggression to smaller than larger heterospecifics when exposed to elevated salinity alone, while G. holbrooki showed no such preference. These results highlight the importance of considering the interplay between multiple abiotic stressors and behavioral interactions between invasive and native species, combined with the modulating effect of species-specific and size based responses to those stressors.

Phylogenetic Relatedness Influences Plant Interspecific Interactions Across Stress Levels in Coastal Ecosystems: a Meta-Analysis

Positive and negative interactions can occur simultaneously between plant species. According to the stress gradient hypothesis (SGH), species interactions shift towards more facilitative interactions or reductions in competition with increasing stress, whereas debate continues over whether evolutionary history influences the strength of species interactions. However, few studies have investigated the effects of phylogenetic relatedness (i.e., the sum of branch lengths separating species on a phylogeny) on the outcomes of interspecific interactions across stress levels. Therefore, we conducted a Bayesian meta-analysis on data collected from publications on plant interactions within coastal ecosystems in order to investigate the effects of phylogenetic relatedness on interspecific interactions across different stress levels. These analyses showed the effect sizes of species interactions on survival and growth to increase with stress increment, supporting the SGH in coastal ecosystems. However, phylogenetic relatedness did not lead to these differences of interspecific interactions between low and high stress. We found that species interactions affecting plant survival were not significantly influenced by phylogenetic relatedness; however, when evolutionary relationships of target and neighbor species were more phylogenetically distant, their interactions were more likely to facilitate growth of target species. Furthermore, the effect of the interaction between phylogenetic distance and stress on species interactions was negative. This suggests the observed net effects of phylogenetically distant neighbor species on target species were not due to true facilitation but reductions in competition when moving from low stress to high stress environments. According to these results, phylogenetic relatedness should be considered in choosing species for restoration of coastal ecosystem plant communities. Specifically, increasing the phylogenetic breadth of the assemblage is more likely to include species that have evolved to reduce stress on surrounding species through modification of the environment.

Biotic forcing: the push–pull of plant ranges

Scientists now recognize the importance of species interactions for range shifts, but lack general predictions about when and how species interactions influence shifts. The ‘biotic envelopes’ of plant species are defined by inter-specific interactions that influence their range limits. Two prominent hypotheses describe the biotic envelopes of plants by predicting that the outcome of inter-specific interactions is determined by climate, especially temperature and aridity. The first hypothesis posits that species distributions are structured by a trade-off between competitive ability and cold tolerance, so plant species exposed to warming climates will have trailing range edges that are limited by competitive interactions. The second hypothesis proposes that the effects of competition and facilitation from neighbouring plants change within a species range, such that facilitative interactions dominate in more environmentally stressful conditions; these facilitative interactions define leading range edges in a warming climate. We incorporate these hypotheses into a common framework that allows us to identify when mismatches in dispersal rates will lead to range expansion or contraction for a focal species. We provide general predictions about the biotic envelopes of plants, and how climate change will alter these envelopes, while highlighting uncertainties in applying these predictions beyond range edges.

Functional trait differences and the outcome of community assembly: an experimental test with vernal pool annual plants

Functional trait‐based approaches have seen rapid development in community ecology and biogeography in recent years, as they promise to offer a better mechanistic and predictive understanding of community structure. However, several key challenges remain. First, while many studies have explored connections between functional traits and abiotic gradients, far fewer have directly tested the common assumption that functional trait differences influence interspecific interactions. Second, empirical studies often ignore intraspecific trait variation within communities, even though intraspecific variation has been known to have substantial impacts on community dynamics. Here we present an experiment designed to assess the role of functional trait differences in predicting the outcome of interspecific species interactions among a suite of California vernal pool annual plants. Eight species were grown in pairwise combinations in two levels of inundation in a greenhouse and functional traits were measured on all individuals. Nested models predicting focal plant performance were fit to the data. For seven of the eight species in the experiment, the best model included a functional trait difference term that was consistent with a competitive hierarchy, indicating that focal species tended to do better when they had larger leaf size, lower specific leaf area, and greater investment in lateral canopy spread than their neighbors. Models that included individually measured trait values generally performed better than models using species trait averages. We tested if the same trait measurements predicted tolerance of inundation (a feature of vernal pool habitats), and species depth distributions from extensive field surveys, though we did not find strong relationships. Our results suggest that functional traits can be used to make inferences about the outcome of interspecific interactions, and that greater predictive power can come from considering intraspecific variation in functional traits, particularly in low diversity communities.

Insect herbivores change the outcome of plant competition through both inter‐ and intraspecific processes

Insect herbivores can affect plant abundance and community composition, and theory suggests that herbivores influence plant communities by altering interspecific interactions among plants. Because the outcome of interspecific interactions is influenced by the per capita competitive ability of plants, density dependence, and intrinsic rates of increase, measuring herbivore effects on all these processes is necessary to understand the mechanisms by which herbivores influence plant communities. We fit alternative competition models to data from a response surface experiment conducted over four years to examine how herbivores affected the outcome of competition between two perennial plants, Solidago altissima and Solanum carolinense. Within a growing season, herbivores reduced S. carolinense plant size but did not affect the size of S. altissima, which exhibited compensatory growth. Across seasons, herbivores did not affect S. carolinense density or biomass but reduced both the density and population growth of S. altissima. The best-fit models indicated that the effects of herbivores varied with year. In some years, herbivores increased the per capita competitive effect of S. altissima on S. carolinense; in other years, herbivores influenced the intrinsic rate of increase of S. altissima. We examined possible herbivore effects on the longer-term outcome of competition (over the time scale of a typical old-field habitat), using simulations based on the best-fit models. In the absence of herbivores, plant coexistence was observed. In the presence of herbivores, S. carolinense was excluded by S. altissima in 72.3% of the simulations. We demonstrate that herbivores can influence the outcome of competition through changes in both per capita competitive effects and intrinsic rates of increase. We discuss the implications of these results for ecological succession and biocontrol.

Insecticide-Mediated Apparent Displacement between Two Invasive Species of Leafminer Fly

BackgroundClosely related invasive species may often displace one another, but it is often difficult to determine mechanisms because of the historical nature of these events. The leafmining flies Liriomyza sativae and Liriomyza trifolii have become serious invasive agricultural pests throughout the world. Where both species have invaded the same region, one predominates over the other. Although L. sativae invaded Hainan Island of China first, it recently has been displaced by the newly invasive L. trifolii. We hypothesized that differential susceptibilities to insecticides could be causing this demographic shift.Methodology/Principal FindingsAvermectin and cyromazine are the most commonly used insecticides to manage leafminers, with laboratory bioassays demonstrating that L. trifolii is significantly less susceptible to these key insecticides than is L. sativae. In trials where similar numbers of larvae of both species infested plants, which subsequently were treated with the insecticides, the eclosing adults were predominately L. trifolii, yet similar numbers of adults of both species eclosed from control plants. The species composition was then surveyed in two regions where L. trifolii has just begun to invade and both species are still common. In field trials, both species occurred in similar proportions before insecticide treatments began. Following applications of avermectin and cyromazine, almost all eclosing adults were L. trifolii in those treatment plots. In control plots, similar numbers of adults of the two species eclosed, lending further credence to the hypothesis that differential insecticide susceptibilities could be driving the ongoing displacement of L. sativae by L. trifolii.Conclusions/SignificanceOur results show that differential insecticide susceptibility can lead to rapid shifts in the demographics of pest complexes. Thus, successful pest management requires the identification of pest species to understand the outcome of insecticide applications. These results further demonstrate the importance of considering anthropogenic factors in the outcome of interspecific interactions.

Distinctive life traits and distribution along environmental gradients of dominant and subordinate Mediterranean ant species

For most animal and plant species, life traits strongly affect their species-specific role, function or position within ecological communities. Previous studies on ant communities have mostly focused on the role of dominant species and the outcome of interspecific interactions. However, life traits of ant species have seldom been considered within a community framework. This study (1) analyses life traits related to ecological and behavioural characteristics of dominant and subordinate ant species from 13 sites distributed throughout the Iberian Peninsula, (2) determines how similar the ant species are within each of the two levels of the dominance hierarchy, and (3) establishes the distribution patterns of these different groups of species along environmental gradients. Our results showed that the differences between dominants and subordinates fall into two main categories: resource exploitation and thermal tolerance. Dominant species have more populated colonies and defend food resources more fiercely than subordinates, but they display low tolerance to high temperatures. We have identified different assemblages of species included within each of these two levels in the dominance hierarchy. The distribution of these assemblages varied along the environmental gradient, shifting from dominant Dolichoderinae and cryptic species in moist areas, to dominant Myrmicinae and hot climate specialists mainly in open and hot sites. We have been able to identify a set of life traits of the most common Iberian ant species that has enabled us to characterise groups of dominant and subordinate species. Although certain common features within the groups of both dominants and subordinates always emerge, other different features allow for differentiating subgroups within each of these groups. These different traits allow the different subgroups coping with particular conditions across environmental gradients.

Geographic variation in a facultative mutualism: consequences for local arthropod composition and diversity

Geographic variation in the outcome of interspecific interactions may influence not only the evolutionary trajectories of species but also the structure of local communities. We investigated this community consequence of geographic variation for a facultative mutualism between ants and wild cotton (Gossypium thurberi). Ants consume wild cotton extrafloral nectar and can protect plants from herbivores. We chose three sites that differed in interaction outcome, including a mutualism (ants provided the greatest benefits to plant fitness and responded to manipulations of extrafloral nectar), a potential commensalism (ants increased plant fitness but were unresponsive to extrafloral nectar), and a neutral interaction (ants neither affected plant fitness nor responded to extrafloral nectar). At all sites, we manipulated ants and extrafloral nectar in a factorial design and monitored the abundance, diversity, and composition of other arthropods occurring on wild cotton plants. We predicted that the effects of ants and extrafloral nectar on arthropods would be largest in the location with the mutualism and weakest where the interaction was neutral. A non-metric multidimensional scaling analysis revealed that the presence of ants altered arthropod composition, but only at the two sites in which ants increased plant fitness. At the site with the mutualism, ants also suppressed detritivore/scavenger abundance and increased aphids. The presence of extrafloral nectar increased arthropod abundance where mutual benefits were the strongest, whereas both arthropod abundance and morphospecies richness declined with extrafloral nectar availability at the site with the weakest ant-plant interaction. Some responses were geographically invariable: total arthropod richness and evenness declined by approximately 20% on plants with ants, and extrafloral nectar reduced carnivore abundance when ants were excluded from plants. These results demonstrate that a facultative ant-plant mutualism can alter the composition of arthropod assemblages on plants and that these community-level consequences vary across the landscape.

Consumer‐resource theory predicts dynamic transitions between outcomes of interspecific interactions

Interactions between two populations are often defined by their interaction outcomes; that is, the positive, neutral, or negative effects of species on one another. Yet, signs of outcomes are not absolute, but vary with the biotic and abiotic contexts of interactions. Here, we develop a general theory for transitions between outcomes based on consumer-resource (C-R) interactions in which one or both species exploit the other as a resource. Simple models of C-R interactions revealed multiple equilibria, including one for species coexistence and others for extinction of one or both species, indicating that species' densities alone could determine the fate of interactions. All possible outcomes [(+ +), (+ -), (--), (+ 0), (- 0), (0 0)] of species coexistence emerged merely through changes in parameter values of C-R interactions, indicating that variation in C-R interactions resulting from biotic and abiotic conditions could determine shifts in outcomes. These results suggest that C-R interactions can provide a broad mechanism for understanding context- and density-dependent transitions between interaction outcomes.

Edaphic quality and plant–pathogen interactions: effects of soil calcium on fungal infection of a serpentine flax

Spatial variation in the frequency and outcome of interspecific interactions is thought to play a central role in shaping geographic patterns of biodiversity. Previous empirical studies and modeling exercises suggest that negative species interactions should occur with greater frequency and intensity in high-quality/low-stress environments. I tested this hypothesis through a four-year epidemiological study of interactions between the plant Hesperolinon californicum and the pathogenic rust fungus Melampsora lini. By virtue of its association with serpentine soils, H. californicum is exposed to edaphic conditions that are stressful to most plants. Notable among these is the low availability of calcium, an element that plays critically important roles in the immune responses of plants to attacking pathogens. As a serpentine generalist, however, H. californicum grows in soils with a wide range of calcium concentrations, and this should lead to differences in the frequency and/or severity of rust infection among host populations. I investigated geographic variation in disease pressure by conducting annual surveys in 16 populations spanning the host's distribution and representing a range of soil calcium concentrations. Results indicated that plants growing in more stressful low-calcium soils experienced higher rates of rust infection, suggesting that soil calcium may modulate host susceptibility in a manner opposite to that predicted by the a priori hypothesis. Epidemiological surveys further revealed a latitudinal cline in disease prevalence, with high infection rates in northern host populations decreasing gradually toward the south. Studies of the fitness effects of disease demonstrated that rust infection caused significant reductions in host survival and fecundity, and there was evidence of a demographic feedback between infection prevalence and host density across survey years.

Ecological dominance of the red imported fire ant, Solenopsis invicta, in its native range

Despite the widespread impacts invasive species can have in introduced populations, little is known about competitive mechanisms and dominance hierarchies between invaders and similar taxa in their native range. This study examines interactions between the red imported fire ant, Solenopsis invicta, and other above-ground foraging ants in two habitats in northeastern Argentina. A combination of pitfall traps and baits was used to characterize the ant communities, their dominance relationships, and to evaluate the effect of phorid flies on the interactions. Twenty-eight ant species coexisted with S. invicta in a gallery forest gap, whereas only ten coexisted with S. invicta in a xerophytic forest grassland. S. invicta was the most numerically dominant species in the richest and complex habitat (gallery forest); however it performed better as discoverer and dominator in the simpler habitat. S. invicta was active during day and night. In spite of its poor capacity to discover resources, S. invicta showed the highest ecological dominance and the second-best behavioral dominance after Camponotus blandus. S. invicta won 78% of the interactions with other ants, mostly against its most frequent competitor, Pheidole cf. obscurithorax, dominating baits via mass recruitment and chemical aggression. P. cf. obscurithorax was the best food discoverer. S. invicta won 80% of the scarce interactions with Linepithema humile. Crematogaster quadriformis was one of the fastest foragers and the only ant that won an equal number of contests against S. invicta. The low presence of phorid flies affected the foraging rate of S. invicta, but not the outcome of interspecific interactions. This study revealed that the red imported fire ant ecologically dominated other terrestrial ants in its native range; however, other species were able to be numerically dominant or co-dominant in its presence.

Distinct habitat types arise along a continuous hydrodynamic stress gradient due to interplay of competition and facilitation

Though species interactions across local environmental gradients are well studied, the way in which species interactions shift between different habitats on a landscape scale has received less attention. We hypothesised that interactions among a suite of shoreline plant species shift across a hydrodynamic-exposure gradient, leading to generation of apparently distinct habitat types (e.g. bare cobble beaches, vegetated cobble beaches, fringing marshes and salt marshes). We examined hydrodynamic forcing and found that it was strongly correlated with shoreline habitat type. A trans- plant experiment revealed that all plants were rapidly crushed and abraded in bare cobble areas with high hydrodynamic energy and were out-competed by grasses in the low-energy salt marshes. Vegetated cobble beaches are inhabited by the largest amount of plant species under intermediate conditions, where hydrodynamic energy is sufficiently low to allow the establishment of the ecosys- tem engineer Spartina and sufficiently high to prevent the monopolisation of space by competitively dominant, but stress intolerant grasses. Experimentally reducing physical and biotic stresses (hydro- dynamics and interspecific competition) on bare cobble beaches and salt marshes, respectively, enabled forbs to persist across the whole gradient. These results demonstrate that the outcome of interspecific interactions at landscape scales is driven by background physical conditions, and that this can result in the development of what are considered distinct habitats.

Interspecific conflict in lizards: Social dominance depends upon an individual's species not its body size

Abstract Animals frequently compete for resources (food, shelter, etc.) against conspecifics as well as against individuals of other species; larger animals typically are dominant over smaller ones. Although body size thus correlates with social dominance in interspecific as well as intraspecific encounters, the causal connection remains unclear. That is, one species might dominate another not simply because of larger size, but some other species‐specific attribute. In such a case, we expect an animal of the subordinate taxon to be subservient to all members of the ‘dominant’ species, even those too small to pose a physical threat. This scenario can be tested by staging encounters over resource use in the laboratory. We conducted such trials using a guild of sympatric montane lizards (Reptilia: Scincidae), that compete for shelter sites (rock crevices), with larger species routinely displacing smaller taxa. Remarkably, neonates of larger species effectively deterred adults of smaller species from entering occupied retreat sites – even when the neonates were much smaller than the adults they displaced. Thus, the outcomes of interspecific interactions in this system depend upon the species of the participants, not their relative body sizes. Measurements of bite force confirm that the neonates of the most dominant species posed little physical threat to heterospecific adults, so that species‐specific variation in fighting ability cannot explain this puzzling result. However, juveniles often share shelter sites with conspecific adults, so that avoidance of neonates may reduce the risk of attack by an unseen adult resident.

CONVERGENT PATTERNS IN THE SELECTION MOSAIC FOR TWO NORTH AMERICAN BIRD‐DISPERSED PINES

The strength and outcome of interspecific interactions often vary across the landscape because of differences in community context. We investigated how the presence or absence of pine squirrels (Tamiasciurus spp.) influences the ecology and (co)evolution of seed‐dispersal mutualisms between Clark's Nutcrackers (Nucifraga columbiana) and limber (Pinus flexilis) and whitebark (P. albicaulis) pines. Nutcrackers are the primary seed dispersers of these pines. Therefore both nutcrackers and pines potentially benefit from the evolution of a cone structure that enhances seed harvest by nutcrackers. Pine squirrels are the dominant predispersal seed predator of these pines throughout the Sierra Nevada–Cascades and Rocky Mountains and do not disperse seeds, so that pines benefit from deterring seed harvest by pine squirrels. To determine whether pines have evolved in response to selection by nutcrackers and pine squirrels, we conducted studies in ranges with pine squirrels and in ranges in the Great Basin and northern Montana where populations of the pines have apparently evolved without pine squirrels for 10 000 years or more. Cone evolution was convergent between both pines and among phylogeographically independent populations with and without pine squirrels, consistent with variation in selection by nutcrackers and pine squirrels. Where pine squirrels were present, they out‐competed nutcrackers for seeds, nutcrackers were less abundant, and selection by pine squirrels constrained the evolution of cone and seed traits that facilitate seed dispersal by nutcrackers. In the absence of pine squirrels, nutcrackers were more than twice as abundant, selection by pine squirrels on cone structure was relaxed, and selection on cone structure by nutcrackers resulted in cones that increased the foraging efficiency of nutcrackers and improved their potential for seed dispersal.

Effects of enrichment on protist abundances and bacterial composition in simple microbial communities

We experimentally investigated effects of nutrient enrichment and trophic structure in a microbial food web consisting of mixed bacteria, two bacterivorous ciliates (Tetrahymena sp. and Colpidium sp.) and an omnivorous ciliate (Blepharisma sp.) feeding on both trophic levels. We assembled all possible food webs including one or more of the ciliate species and cross‐classified them with four levels of enrichment of the bacterial medium. The qualitative outcome of food web interactions was independent of enrichment and always the same: Tetrahymena strongly depressed or excluded Colpidium, and Blepharisma strongly depressed or excluded both bacterivores. Consequently, in all sub‐webs only the dominant ciliate species responded positively to enrichment. The total density of bacteria increased with enrichment irrespective of food web composition. In contrast, the response of single‐celled bacteria to enrichment depended on food web composition and was only weakly positive in most food webs with the omnivore. Enrichment had a positive effect on the relative success of (presumably more defended) bacterial aggregates. The outcome of interspecific interactions among ciliates could not be predicted from monoculture experiments and deviated from earlier experiments in which each bacterivore coexisted separately with the omnivore. As a potential explanation we suggest that changes in experimental protocol reduced spatial heterogeneity and increased attack rates. A simple, dynamical model shows that increased attack rates can indeed greatly decrease the upper limit and range of enrichment over which intermediate consumers can coexist with omnivores.

Body size, sexual size dimorphism, and Rensch's rule in adult hydropsychid caddisflies (Trichoptera: Hydropsychidae)

Body size influences most biological processes from metabolic rates to the outcome of interspecific interactions. Within a species, sexual size dimorphism (SSD) reflects either differential selection on body size of males and females or phylogenetic inertia. Among taxa, SSD should decrease as body size increases when females are the larger sex — a pattern known as Rensch's rule. We examined body size, SSD, and Rensch's rule among 29 species of adult hydropsychid caddisflies (Trichoptera: Hydropsychidae) and 12 closely related caddisfly species. Females were almost always larger than males in all species examined. Body size variation among genera was greater than variance among species. In contrast, the greatest variance in SSD was among species within a genus. Contrary to Rensch's rule, the degree of SSD did not change as body size increased among genera. Observed body size patterns suggest that hydropsychid caddisfly species within a genus may be subjected to similar selective pressures during the larval stage, but this issue remains to be investigated. In addition, our data suggest that hydropsychids may violate Rensch's rule, a pattern not often reported. Our data provide a basis for proposing and testing hypotheses about the ecology and evolution of hydropsychid caddisflies.

Geographic Variation in the Behaviour of the Cleaner Fish Labroides dimidiatus (Labridae)

Geographical variation in the outcome of interspecific interactions has a range of proximate ecological causes. For instance, cleaning interactions between coral reef fishes can result in benefits for both the cleaner and its clients. However, because both parties can cheat and because the rewards of cheating may depend on the local abundance of ectoparasites on clients, the interaction might range from exploitative to mutualistic. In a comparative analysis of behavioural measures of the association between the cleaner fish Labroides dimidiatus and all its client species, we compared cleaning interactions between two sites on the Great Barrier Reef that differ with respect to mean ectoparasite abundance. At Heron Island, where client fish consistently harbour fewer ectoparasites, client species that tended to pose for cleaners were more likely to receive feeding bites by cleaners than client species that did not pose for cleaners. This was not the case at Lizard Island, where ectoparasites are significantly more abundant. Client fish generally spent more time posing for cleaners at Lizard Island than their conspecifics at Heron Island. However, fish at Heron Island were inspected longer on average by cleaners than conspecifics at Lizard Island, and they incurred more bites and swipes at their sides per unit time from cleaners. These and other differences between the two sites suggest that the local availability of ectoparasites as a food source for cleaners may determine whether clients will seek cleaning, and whether cleaners will feed on parasites or attempt to feed on client mucus. The results suggest that cleaning symbiosis is a mosaic of different outcomes driven by geographical differences in the benefits for both participants.